Frequency hopping method for RFID tag

ABSTRACT

Radio frequency (RF) power is sent out by a base station to radio frequency identification transponders (RFID tags) for a first time at a first frequency. The frequency is changed to a second frequency, and the RF power sent out for a second time substantially different from the first time.

FIELD OF THE INVENTION

The field of the invention is the field of radio frequency (RF)identification (RFID) transponders (tags), and systems for their use.

BACKGROUND OF THE INVENTION

RF Transponders (RF Tags) can be used in a multiplicity of ways forlocating and identifying accompanying objects and transmittinginformation about the state of the object. It has been known since theearly 60's in U.S. Pat. No. 3,098,971 by R. M. Richardson, thatelectronic components of transponders could be powered by radiofrequency (RF) electromagnetic (EM) waves sent by a “base station” andreceived by a tag antenna on the transponder. The RF EM field induces analternating current in the transponder antenna which can be rectified byan RF diode on the transponder, and the rectified current can be usedfor a power supply for the electronic components of the transponder. Thetransponder antenna loading is changed by something that was to bemeasured, for example a microphone resistance in the cited patent. Theoscillating current induced in the transponder antenna from the incomingRF energy would thus be changed, and the change in the oscillatingcurrent led to a change in the RF power radiated from the transponderantenna. This change in the radiated power from the transponder antennacould be picked up by the base station antenna and thus the microphonewould in effect broadcast power without itself having a self containedpower supply. The “rebroadcast” of the incoming RF energy isconventionally called “back scattering”, even though the transponderbroadcasts the energy in a pattern determined solely by the transponderantenna. Since this type of transponder carries no source of energy ofits own, it is called a “passive” transponder to distinguish it from atransponder containing a battery or other energy supply, conventionallycalled an active transponder. The power supply of the passivetransponder is typically a capacitor which is charged by rectifying theRF power signal sent out by the base station, but may be any source ofpower which is energized by an external signal.

Active transponders with batteries or other independent energy storageand supply means such as fuel cells, solar cells, radioactive energysources etc. can carry enough energy to energize logic, memory, and tagantenna control circuits. However, the usual problems with life andexpense limit the usefulness of such transponders.

In the 70's, suggestions to use backscatter transponders with memorieswere made. In this way, the transponder could not only be used tomeasure some characteristic, for example the temperature of an animal inU.S. Pat. No. 4,075,632 to Baldwin et. al., but could also identify theanimal.

The continuing march of semiconductor technology to smaller, faster, andless power hungry has allowed enormous increases of function andenormous drop of cost of such transponders. Presently available researchand development technology will also allow new function and differentproducts in communications technology. However, the new functionsallowed and desired consume more and more power, even though theindividual components consume less power.

It is thus of increasing importance to be able to power the transpondersadequately and increase the range which at which they can be used. Onemethod of powering the transponders suggested is to send informationback and forth to the transponder using normal RF techniques and totransport power by some means other than the RF power at thecommunications frequency. However, such means require use of possiblytwo tag antennas or more complicated electronics.

Sending a swept frequency to a transponder was suggested in U.S. Pat.No. 3,774,205. The transponder would have elements resonant at differentfrequencies connected to the tag antenna, so that when the frequencyswept over one of the resonances, the tag antenna response would change,and the backscattered signal could be picked up and the resonancepattern detected.

Prior art systems can interrogate the tags if more than one tag is inthe field. U.S. Pat. No. 5,214,410, hereby incorporated by reference,teaches a method for a base station to communicate with a plurality oftags.

Sending at least two frequencies from at least two antennas to avoid the“dead spots” caused by reflection of the RF was proposed in EPO 598 624A1, by Marsh et al. The two frequencies would be transmittedsimultaneously, so that a transponder in the “dead spot” of onefrequency would never be without power and lose its memory of thepreceding transaction.

The prior art teaches a method to interrogate a plurality of tags in thefield of the base station. The tags are energized, and send a responsesignal at random times. If the base station can read a tag unimpeded bysignals from other tags, the base station interrupts the interrogationsignal, and the tag which is sending and has been identified, shutsdown. The process continues until all tags in the field have beenidentified. If the number of possible tags in the field is large, thisprocess can take a very long time. The average time between the randomresponses of the tags must be set very long so that there is areasonable probability that a tag can communicate in a time window freeof interference from the other tags.

In order that the prior art methods of communicating with a multiplicityof tags can be carried out, it is important that the tags continue toreceive power for the tag electronics during the entire communicationperiod. If the power reception is interrupted for a length of time whichexceeds the energy storage time of the tag power supply, the tag “loses”the memory that it was turned off from communication, and will restarttrying to communicate with the base station, and interfere with theorderly communication between the base station and the multiplicity oftags.

The amount of power that can be broadcast in each RF band is severelylimited by law and regulation to avoid interference between two users ofthe electromagnetic spectrum. For some particular RF bands, there aretwo limits on the power radiated. One limit is a limit on thecontinuously radiated power in a particular bandwidth, and another limitis a limit on peak power. The amount of power that can be pulsed in aparticular frequency band for a short time is much higher than thatwhich can be broadcast continuously.

Federal Communications Commission Regulation 15.247 and 15.249 of Apr.25, 1989 (47 C.F.R. 15.247 and 15.249) regulates the communicationstransmissions on bands 902-928 MHZ, 2400-2483.5 MHZ, and 5725-5850 MHZ.In this section, intentional communications transmitters are allowed tocommunicate to a receiver by frequently changing frequencies on both thetransmitter and the receiver in synchronism or by “spreading out” thepower over a broader bandwidth. The receiver is, however, required tochange the reception frequency in synchronism with the transmitter.

RELATED PATENTS AND APPLICATIONS

The following U.S. Patents and Patent Applications are assigned to theassignee of the present invention: U.S. Pat. Nos.: 6,320,896, 6,327,312,6,005,530, 6,122,329, 6,501,807, 6,294,997, 6,166,638, 6,441,740,6,104,291, 5,939,984, 6,140,146, 6,259,408, 6,236,223, 6,249,227,6,201,474, 6,100,804, 6,294,996, 6,486,769, 6,121,880, 6,518,885,6,593,845, 6,320,509, 6,639,509, 5,485,520, 6,275,157, 6,285,342,6,366,260, 6,215,402, 6,118,379, 6,177,872, 6,281,794, 6,130,612,6,147,606, 6,288,629, 6,172,596, 6,566,850, 6,535,175; 5,850,181;5,828,693;; and U.S. patent application Ser. Nos. 09/394,241 filed Sep.13, 1999, 10/056,398 filed Jan. 23, 2002, and 60/459,414 filed Mar. 31,2003. The above patents and patent applications are hereby incorporatedby reference.

OBJECTS OF THE INVENTION

It is an object of the invention to produce a method, an apparatus, anda system communicating between a base station and at least one tag whichdecreases the time taken to identify the tag or tags.

SUMMARY OF THE INVENTION

Information is communicated between a base station and at least one tagby sending RF power P_(j) for a first time t_(j) to the tag at a firstfrequency ƒ_(j) from the base station to the tag, then sending power fora second time t_(k) to the tag at a second frequency ƒ_(k), where t_(j)and t_(k) are substantially different times.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is the power and FIG. 1B is the frequency transmitted as afunction of time in the prior art.

FIG. 2A is the power and FIG. 2B is the frequency transmitted as afunction of time in one of the preferred methods of the invention.

FIG. 3 is block diagram of a preferred method of the invention.

DETAILED DESCRIPTION OF THE INVENTION

U.S. Pat. No. 5,828,693 to Mays, et al. issued Oct. 27, 1998 entitledSpread spectrum frequency hopping reader system and U.S. Pat. No.5,850,181 to Heinrich, et al. issued Dec. 15, 1998 entitled Method oftransporting radio frequency power to energize radio frequencyidentification transponders, assigned to the assignee of the presentinvention, give details on RFID tags powered by an RF field where thefrequency sent to the tags hops from frequency to frequency chosen froma pseudorandomly ordered list of frequencies. In both the abovedescribed patents, the RF field is sent out to the tags from a basestation as a series of bursts of power at a particular frequency, withthe frequency changing for the next burst, but the power and the lengthof time of the bursts are kept constant. U.S. Pat. No. 5,828,693 teachesthat the length of time of each burst the regular series of bursts maybe changed to avoid having one or more base stations interfering withone another. Apparatus and methods for changing the frequency and thepower sent out by the tags are well described in these patents. Theabove patents are hereby incorporated by reference.

In a preferred communication between a base station and a group of tags,each tag is identified, and then instructed to take no further part inthe communication unless it is called upon to do so by calling itsidentification number. Since two tags “talking” at the same time to thebase station will interfere with each other, a tag which has once beenidentified, and which loses its “memory” that it was identified, willslow the communication with the group down because it will have to bere-identified and re-instructed to keep silence. In the U.S. Pat. No.5,850,181 referred to above, the importance of keeping the tagfunctional by not allowing the power in the tag to drop below a minimumwas pointed out. In a preferred embodiment, well described in copendingapplication Ser. No. 10/056,398 assigned to the assignee of the presentinvention filed Jan. 23, 2002 by Heinrich et al., power is provided fora long time t₀ to just one device or function on the tag . . . thedevice or “flag” which tells the tag that it has been identified. Aseparate power supply such as a capacitor is provided which providespower only to the flag for a time t₀ long compared to the normal tagpower down time when all the tag electronics are drawing current (whichcould be as short at 50 microsec). Such a situation may occur, forexample, when the frequency sent to the tag changes, and the tag is in aposition where multipath effects drop the power received by the alreadyidentified tag below that power which the tag needs to be fullyfunctional. If the tag flag remains set until the frequency is changedagain and the multipath transmission changes so the tag is powered onceagain, the tag remembers that it has been identified, and does notinterrupt communications by trying to contact the base station. Theabove application Ser. No. 10/056,398 is hereby incorporated byreference.

When a group of tags is being interrogated by a base station, the basestation according to the prior art sends out signals at a frequencyƒ_(i) for a fixed time t_(i), and then changes frequency to anotherfrequency ƒ_(j) chosen from a list of frequencies listed in pseudorandomorder, and then sends frequency ƒ_(j) for the same time t_(i). Thisprocess is continued until all tags have been identified. It may be,however, that the base station sends out a command for unidentified tagsin the field to respond, and no tags respond, either because all tags inthe field have been identified or because some tags in the field do notreceive power because of the above identified multipath problems.Presently, the base station continues to send power at the samefrequency and power for the same amount of time regardless of whether atag in the field responds. The base station continues through thepseudorandomly ordered list of frequencies, and either stopstransmission or starts again at the beginning of the list. U.S. Pat. No.5,828,693 mentions that the amount of time that a base station sends outa particular frequency before the frequency changes may be changed, butdoes not state conditions for such changes. In particular, U.S. Pat. No.5,828,693 does not specify that the length of time taken to change thetime interval shall be less than the time taken to power down the tag orthe time for the flag to reset.

In the most preferred method of the present invention, the base stationchanges frequency as soon as no tags respond, so that those unidentifiedtags which are silent because they are in a multipath power minimum atfrequency ƒ_(j) will see a different frequency ƒ_(j+1), for which themultipath minima are in a different spatial positions. For example, at2.4 GHz, the frequency might be changed in the prior art every 300 or400 msec. However, the base station can tell if one or more tags isresponding in as little as 10 ms. Thus, the base station will changefrequencies in as little as 10 or 20 ms as soon as no more tags respond.Preferably, when the time is changed from a time t_(j) to another timet_(j+1), t_(j+1) will be less than t_(j)/2. More preferably, t_(j+1)will be less than t_(j)/4, and most preferably t_(j+1) will be less thant_(j)/10. To take into account that t_(j+1) may also be longer thant_(j), preferably |t_(j+1)−t_(j)|>0.05 (t_(j)+t_(j+1)), more preferably|t_(j+1)−t_(j)|>0.1 (t_(j)+t_(j+1)) and most preferably|t_(j+1)−t_(j)|>0.3 (t_(j)+t_(j+1)).

FIGS. 1A and 1B show the prior art sent out RF power and frequency as afunction of time. The frequency is changed at regular times, and thepower is greatly reduced as the frequency is changed. FIG. 2A shows asketch of RF power as a function of time for the method of theinvention. After sending out a power P_(i) at a frequency ƒ_(i) for atime t_(i), the frequency is changed and a new frequency chosen in orderfrom a list of frequencies listed in pseudorandom order. Instead ofsending a new frequency ƒ_(j) for the same time t_(i), the frequencyƒ_(j) is sent out for a time t_(j) which is substantially different fromt_(i). The time taken to change the frequency from ƒ_(i) to ƒ_(j) andthe timing from t_(i) to t_(j) must be less than the time t₀ for the tagflag to be reset, and is preferably less than the time taken for the tagto power down once the RF field drops to zero. While the power levelssent out in FIG. 2A are shown to be constant with time, the inventionanticipates that the power level sent out may change as a function oftime. The power level may be an increasing or decreasing stairstepfunction, or indeed any regular function of time.

FIG. 3 shows a block diagram of the most preferred method of theinvention. The base station starts by choosing the first frequency inthe ordered list and sets j=1 in step 300. Then, the base station sendsout RF energy a frequency ƒ_(j) for a time sufficient for a single tagto respond in step 310. In decision step 320, the base station decideswhether one or more tags responded. If one or more tags responded,another decision step 320 decides whether the total time t_(j) spentsending out frequency ƒ_(j) exceeds a maximum time limit t_(max) forsending out a single frequency at the power sent. Government regulationsprohibit power of over a certain limit being sent out for more than adefined time. The protocol sets a maximum time limit t_(max) (which mayoptionally depend on power sent out) for sending out one frequency, andwhen that time limit has been exceeded, the index j is changed to j+1 instep 340, and the system returns to step 310 to send out another thenext frequency ƒ_(j+1) in the lists. If no tags responded in step 320,the system goes immediately to step 340 and to change frequency to thenext frequency ƒ_(j+1) in the list.

In the most preferred method of the invention, the maximum time t_(max)for sending out a single frequency may be reached while the firstfrequency is being sent out, since there are many unread tags in thefield. Eventually, however, most tags have been read, and at that time,no tags return signals before the maximum time t_(max) has been reached.Then, the base station cycles through the remaining frequencies in thelist, or the base station decides that all tags have been identified,and starts the remainder of the protocol for communicating with thetags. It is anticipated by the inventors that the time for sending outthe frequency f_(j+1) in the list of frequencies could in fact be longerthan the time for sending out the prior frequency f_(j), as new tagscould move into the field during the communication procedure.

It is anticipated by the inventors that the base station could send outvarious power levels during the communication, since fewer tags would bein effective communication with the base station if the sent out powerwas lower, and hence the fewer tags could be identified rapidly. Then,the power could be raised to “catch” more of the tags in the field.Alternatively, the power could be sent out high at first, and if morethan one tag responds the power could be reduced to reduce the number oftags in effective communication with the base station.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

1. A method, comprising: sending power to at least one radio frequency(RF) identification (RFID) transponder (tag) by; a) sending power P_(j)for a first time t_(j) to the tag at a first frequency ƒ_(j) chosen froma list of N frequencies ƒ₁ . . . ƒ_(j), ƒ_(j+1) . . . ƒ_(N); and then b)sending power P_(j+1) for a time t_(j+1) to the tag at a secondfrequency ƒ_(j+1) chosen from the list of N frequencies, wherein t_(j)and t_(j+1) are substantially different times, and wherein the timebetween sending power P_(j) and P_(j+1) is less than a time t₀ in whichthe tag loses a particular tag function if no power is sent to the tag.2. The method of claim 1, wherein t_(j+1) is chosen to be long enoughthat all tags in operative communication with the base station atfrequency ƒ_(j+1) have identifed themselves.
 3. The method of claim 1,wherein the sending of power P_(j+1) is stopped after a time t_(j+1)when no further tags identify themselves.
 4. The method of claim 1,wherein P_(j) and P_(j+1) are substantially different powers.
 5. Themethod of claim 4, wherein P_(j+1) is substantially reduced from P_(j)when t_(j) is too short a time for all tags in operative communicationwith the base station to identified themselves.
 6. The method of claim1, wherein |t_(j+1)−t_(j)|>0.05 (t_(j)+t_(j+1)).
 7. The method of claim6, wherein |t_(j+1)−t_(j)|>0.1 (t_(j)+t_(j+1)).
 8. The method of claim7, wherein |t_(j+1)−t_(j)|>0.3 (t_(j)+t_(j+1)).
 9. The method of claim1, wherein P_(j) is a function of time.
 10. The method of claim 9,wherein P_(j) is a monotonically increasing function of time.
 11. Themethod of claim 10, wherein P_(j) is increased when no further tagsidentify themselves.